In this century, the shortage of fresh water will surpass the shortage of energy as a global concern for humanity; and these two challenges are inexorably linked, as explained, for example, in the “Special Report on Water” in the 20 May 2010 issue of The Economist. Fresh water is one of the most fundamental needs of humans and other organisms; each human needs to consume a minimum of about two liters per day. The world also faces greater freshwater demands from farming and industrial processes.
The hazards posed by insufficient water supplies are particularly acute. A shortage of fresh water may lead to a variety of crises, including famine, disease, death, forced mass migration, cross-region conflict/war, and collapsed ecosystems. Despite the criticality of the need for fresh water and the profound consequences of shortages, supplies of fresh water are particularly constrained. 97.5% of the water on Earth is salty, and about 70% of the remainder is locked up as ice (mostly in ice caps and glaciers), leaving only a fraction of all water on Earth as available fresh (non-saline) water.
Moreover, the earth's water that is fresh and available is not evenly distributed. For example, heavily populated countries, such as India and China, have many regions that are subject to scarce supplies. Further still, the supply of fresh water is often seasonally inconsistent. Meanwhile, demands for fresh water are tightening across the globe. Reservoirs are drying up; aquifers are falling; rivers are dying; and glaciers and ice caps are retracting. Rising populations increase demand, as do shifts in farming and increased industrialization. Climate change poses even more threats in many regions. Consequently, the number of people facing water shortages is increasing. Naturally occurring fresh water, however, is typically confined to regional drainage basins; and transport of water is expensive and energy-intensive. Nevertheless, many of the existing processes for producing fresh water from seawater (or from brackish water or contaminated waste streams) require massive amounts of energy. Reverse osmosis (RO) is currently the leading desalination technology. In large-scale plants, the specific electricity required can be as low as 4 kWh/m3 at 30% recovery, compared to the theoretical minimum of around 1 kWh/m3; smaller-scale RO systems (e.g., aboard ships) are less efficient. Other existing seawater desalination systems include thermal-energy-based multi-stage flash (MSF) distillation, and multi-effect distillation (MED), both of which are energy- and capital-intensive processes. In MSF and MED systems, however, the maximum brine temperature and the maximum temperature of the heat input are limited in order to avoid calcium sulfate, magnesium hydroxide and calcium carbonate precipitation, which leads to the formation of soft and hard scale on the heat transfer equipment.
Humidification-dehumidification (HDH) desalination systems include a humidifier and a dehumidifier as their main components and use a carrier gas (e.g., air) to communicate energy between the heat source and the brine. A simple version of this technology includes a humidifier, a dehumidifier, and a heater to heat the seawater stream. In the humidifier, hot seawater comes in direct contact with dry air, and this air becomes heated and humidified. In the dehumidifier, the heated and humidified air is brought into (indirect) contact with cold seawater and gets dehumidified, producing pure water and dehumidified air. As with MSF and MED systems, precipitation of scaling components can occur within the system with consequent damage if the temperature rises too high.
Another approach, described in U.S. Pat. No. 8,119,007 B2 (A. Bajpayee, et al.), uses directional solvent that directionally dissolves water but does not dissolve salt. The directional solvent is heated to dissolve water from a salt solution into the directional solvent. The remaining highly concentrated salt water is removed, and the solution of directional solvent and water is cooled to precipitate substantially pure water out of the solution.
Some of the present inventors were also named as inventors on the following patent applications that include additional discussion of HDH and other processes for purifying water: U.S. application Ser. No. 12/554,726, filed 4 Sep. 2009 now U.S. Pat. No. 8,465,006 B2); U.S. application Ser. No. 12/573,221, filed 5 Oct. 2009 (now U.S. Pat. No. 8,252,092 B2); U.S. application Ser. No. 13/028,170, filed 15 Feb. 2011 (published as U.S.2012/0205236 A1); and U.S. application Ser. No. 13/241,907, filed 23 Sep. 2011 (published as U.S. 2013/0074694 A1); and U.S. application Ser. No. 13/550,094, filed 16 Jul. 2012 (now U.S. Pat. No. 8,496,234 B1).